Answer:The Xiongnu first appear in Chinese historical records about the 5th century BCE, when their repeated invasions prompted the small kingdoms of North China to begin erecting what later became the Great Wall. The Xiongnu became a real threat to China after the 3rd century BCE, when they formed a far-flung tribal confederation under a ruler known as the chanyu, the rough equivalent of the Chinese emperor’s designation as the tianzi (“son of heaven”). They ruled over a territory that extended from western Manchuria (Northeast Provinces) to the Pamirs and covered much of present Siberia and Mongolia. The Xiongnu were fierce mounted warriors who were able to muster as many as 300,000 horseback archers on their periodic intrusions into North China, and they were more than a match for the much less-maneuverable chariots of the Chinese. The completion of the Great Wall along the whole of China’s northern frontier during the Qin dynasty (221–206 BCE) slowed but did not stop the Xiongnu. The early Han dynasty rulers attempted to control them by marrying their leaders to Chinese princesses. But Xiongnu raids against China continued periodically until the Han emperor Wudi (reigned 141/140–87/86 BCE) initiated a fiercely aggressive policy against the nomads, sending expeditions into central China to outflank them and to negotiate alliances with their enemies. These expeditions led to the Chinese conquest of the state of Chosŏn in northern Korea and southern Manchuria and the Chinese exploration of Turkistan.
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